Method for manufacturing a steel for enameling

ABSTRACT

The material is a low-carbon steel having not more than 0.25% by weight of Mn, with the particular relation of Mn, S and O content. It is hot rolled and coiled at 600*C to 800*C. It is then cold rolled and annealed for recrystalization. A product having excellent press formability and availability for enameling is thus obtained.

United States Patent [191 Gondo et a1.

[5 METHOD FOR MANUFACTURING A STEEL FOR ENAMELING [75] Inventors:Hisashi Gondo; Hiroshi Takechi;

Mitsunobu Abe; Kazuo Namba, all of Kisarazu, Japan [73] Assignee: NipponSteel Corporation, Tokyo,

Japan [22] Filed: June 28, 1973 [21] Appl. No.: 374,390

[30] Foreign Application Priority Data u. o coluuc TEMPERATURE (cl 8 A Em 550 g a a u 620 0 670 a; 5,

u- 5 6 700 o I Q '5 n: 4

[ Oct. 14, 1975 3,183,078 5/1965 Ohtake et al. 75/49 3,806,376 4/1974Toda et a1 148/123 UIHER PUBLICATIONS Dewsnap, R. F.; An Investigationof the Rapid Annealing of Sheet Gauge Mild Steel; Special Report 79; pp.112-120; The Iron and Steel Inst., London, 1963.

Primary Examiner-W. Stallard Attorney, Agent, or FirmWatson LeavenworthKelton & Taggart ABSTRACT The material is a low-carbon steel having notmore than 0.25% by weight of Mn, with the particular relation of Mn, Sand 0 content. It is hot rolled and coiled at 600C to 800C. It is thencold rolled and annealed for recrystalization. A product havingexcellent press formability and availability for enameling is thusobtained.

5 Claims, 1 Drawing Figure METHOD FOR MANUFACTURING A STEEL FORENAMELING BACKGROUND OF THE INVENTION It has heretofore been requiredthat the cold-rolled steel for enameling having wide applications invarious household wares, sanitary articles electric products and so onshould have excellent enameling characteristic and press formability.

The availability for enameling can be judged as fish scaling, bubbling,sagging, boiling, which may be caused after enameling. The fish scalingis caused by the hydrogen entrained in the steel sheet during enamelingwhich is diffused over the surface of the sheet as the time elapses,then breaks the film of the enamel and escapes in the air. In thisconnection, it has been reported that if there are more voids presentinside the steel sheet, the fish scaling less occurs since there aremore rooms for the hydrogen to remain.

The most common method to form such voids is to deteriorate the purityof the sheet itself. In other words, it is to increase the non-metallicinclusions therein ,so as to utilize the voids occuring in the coldrolling of the steel sheet due to the difference in deformabilitybetween the inclusions and the others.

This method is, however, undesirable since the fish scaling propertiesmay be improved thereby but the press formability is degraded.

Another effort has been made, wherein the amount of carbon is at firstincreased in the steel sheet and the massive carbide aggregates areformed before the cold rolling step, said aggregates having samefunction as the non-metallic inclusions and thus forming voids, andthereafter decarburization is effected instead of using the non-metallicinclusions. However, this method has also disadvantages that the costfor the decarburization and annealing step is too much expensive andthat the method itself is not suitable to such material having a greatthickness as the bath tub.

The bubbling and the sagging are both the phenomena resulting from theamount of carbon in the steel sheet. Thus, in the former case, the CO orCO gases are generated therefrom, while in the latter case, it isconsidered to be caused by the fact that due to the lowering of thetransformation temperature, the transformation may easily be generated,which results in expansion or contraction of the volume of the material.Accordingly, it is more preferable to have smaller amount of carbon inthe material, although it has been reported that it does not raise muchproblem in a two-coat enameling process.

It is therefore an object of this invention to overcome the defectswhich would be encountered in the prior art.

It is another object of this invention to control the amount of Mn, S, Oto be included in the steel sheet whereby the press formability of thematerial is remarkably improved.

It is still another object of this invention to control the hot rollingconditions before the cold rolling whereby the material as the steelsheet for porcelain enamel is enhanced with respect to the fish scaling.

SUMMARY OF THE INVENTION This invention relates to a method formanufacturing a steel for enameling.

According to this invention, there is provided a method for themanufacture of a cold-rolled lowcarbon steel sheet used for porcelainenamel having excellent press formability and availability for enamelingwhich comprising; providing a low carbon steel material having not morethan 0.25% by weight of manganese; said steel material containing Mn, 0,and S in the relation of where AW atomic weight; subjecting said steelmaterial to hot rolling followed by coiling the same at 600C to 800C;subjecting said coiled hot rolled strip to cold rolling to therebyobtain a cold rolled strip; and thereafter subjecting said cold rolledstrip to recrystalization annealing such as box annealing withover-aging treatment.

The starting material used in this invention is a lowcarbon steel havinglow Mn content. The thin steel sheet to be subjected to press workingusually contains more than about 0.30% Mn. However, an excess amount ofMn exerts an effect upon the mechanical properties of the steel sheet,and in case of the rimmed steel sheet having low-carbon content, it is acommon practice to maintain the Mn content as low as possible so as toobtain good surface qualities. The inventors have now found that anappropriate amount of MnS and MnO which is assured by a low Mn contentis useful for a continuous annealing treatment. In other words, theamount of Mn required in this invention is that necessary to fully fixthe harmful S and O which exists as impurities due to thehigh-temperature coiling of the hot rolled sheet described later. Thatis, the amount by weight of Mn in the steel may be the chemicalequivalent to combine with S and O, or slightly more than that, and itshould be not more than 0.25% and preferably 0.20% or less. Also as tothe contents of Mn, S and 0, it is necessary that K of the followingequation lies in the range of 0.20 5 K S 0.2, and preferably 0.15 5 K S0.15 particularly when the prominent effect of this invention isrequired. It has further been found that the optimum range of K shouldbe 0. 15 to 0 when the excellent enameling characteristic isparticularly required.

AWofMn AWofS If the K is in this range, the average plastic strainratio, (F value), which shows the press formability and deep drawabilityof the steel sheet becomes 1.4 or more as seen from the drawing. It isthus obvious that the steel sheet has extremely good press formabilityand deep drawability. In passing, it is well known that r value dependsupon the texture.

If Mn exists in an amount of 0.3% or more as in the conventional steelsheet, it forms an excessive MnS and MnO even if the equation 0.20 i K 50.20 is satisfied with respect to S% and 0%, and exerts an adverseeffect upon the properties of the steel sheet. This invention has afeature that the relation of Mn, S and O is within the particular rangeas mentioned above, but it also has a feature that after hot rolling,the hot rolled steel sheet is coiled at high temperature for the purposeof converting the harmful S and 0 into the innocent inclusion in thesteel having the particular component range as defined above. Theexistence ofS and O in the steel gives rise to red shortness as well asrestriction of growth of crystal when the annealing for recrystalizationis conducted. Thus, at the time of annealing, a lot of crystal nucleioccurs due to the S inclusion consisting mainly of FeS, which nucleimutually prevent the growth of crystal grains and at the same time the Sinclusion consisting mainly of FeS itself becomes an obstacle to thegrain growth. The 0 gives, as S does, an undesirable factor to thegrowth of crystal and thus, by means of fixing these harmful S and Owith Mn to the condition which is harmless to the grain growth, it ispossible to create the circumstances wherein the S and O are dispersedless densely and the full growth of crystal can be expected.

As set forth herein above, this invention is intended to convert theharmful S and O to the harmless MnS and MnO by means of coiling the hotrolled steel sheet at high temperature, which will make it possible tosuppress the occurrence of the nuclei at the recrystalization annealingso that the circumstances which are favorable for growth of crystal maybe obtained; to accelerate the growth of the crystal grains by means ofthe self-annealing action of the hot rolled coil during the coolingcourse; and thereby to lessen the strain energy after cold rolling andpreferentially enlarge the crystal having orientation capable ofincreasing the Tvalue at the recrystalization annealing. This is notdescribed in this specification but can be confirmed by measurement byX-ray of the texture. In order to obtain such desirable result, thecoiling temperature as mentioned above should be at least 600C, andpreferably at least 650C. However, when the coiling is conducted atextremely high temperature, the difference in temperatures of the hotrolled sheet itself is so severe that the control thereof may becomedifficult. Thus it should be 800 C or less and the preferable range isbetween 650C and 800C.

On the other hand, by means of providing the high coiling temperature atthe hot rolling, it also makes it possible to make the carbides in theagglomeration, globular form. The MnS, MnO carbides thus formed mayoffer an optimum room for making voids at the time of cold rolling,whereby the fish scaling of the steel sheet is considered to beprevented.

BRIEF DESCRIPTION OF THE DRAWING Attached drawing is a diagram showingthe relation of the component ratio of the steel sheet with the averageplastic strain ratio (T) and the occurrence ratio of the fish scaling.

Seeing from the drawing, it is understood that the occurrence ratio ofthe fish scaling depends strongly upon the K and the coilingtemperature. If the coiling temperature is at least 670C and the K iswithin the range between 0.2 and 0.2, the occurrence ratio of the fishscaling can be made zero. The K within this range accords with the rangein which theTvalue is at least 1.4, which shows that it is thepreferable range both for press formability and for availability toenameling.

The occurrence ratio of the fish scaling in the drawing is obtained byenameling 50 specimens each having a size of 10cm X 10cm, subjecting thesame again to the heat treatment at 400C for 30 minutes and comparingthe number of the specimens in which the fish scaling has occurred withothers. Even in case of the steel having K of the range in which theoccurrence ratio of fish scaling is zero in drawing, the fish scalingproperties become much better or occur less often as the K becomessmaller when another more rigid test for juding the fish scalingproperties is conducted. That is, when the hydrogen gas is compulsorilygenerated on one side of the sheet and the time for the hydrogen gas toreach the reverse side of the sheet (the hydrogen pass time) is measuredfor juding the fish scaling properties, it is found that as the Kbecomes smaller the hydrogen pass time becomes longer, and when the Kchanges from plus to minus the hydrogen pass time becomes remarkablylonger.

Generally, the more voids which are sources for absorbing hydrogen, thelonger hydrogen pass time.

Thus, it can be further said that the longer the hydrogen pass time is,the better are the fish scaling properties. The reason why the fishscaling properties become remarkably better particularly in the range ofK being minus may be that the amount of MnS and MnO is in creased withinthe range that they do not harm the properties of the material, whichresult in prominent generation of voids due to the cold rolling.

The specimens in the drawing are manufactured as follows:

A rimmed steel material having composition of C: 0.03 to 0.05%, Mn: 0.14to 0.31%, S: 0.007 to 0.022%, O: 0.010 to 0.085%, the rest:substantially Fe is hot rolled, coiled at 700C to 730C, and then coldrolled to the final thickness (0.8mm). The resulting steel sheet issubjected to the continuous annealing treatment for 700C X 1.5 minutesand then to the overaging treatment for 350C X 5 minutes.

The advantage of the present invention can also be confirmed by usingthe box annealing (700C X 4 hours) after cold rolling and similarpreferable results can be obtained.

The steel material to which the present invention may be applied isgenerally one which is produced by converter or other steelmanufacturing furnaces, and subjected to the steps of casting, blooming,hot-rolling, etc. and further, cold rolling, continuous annealingincluding over-aging or box annealing and, if desired, the temperrolling treatment.

The conditions for those steps may vary as desired in so far as therequirements such as component of the material or the coilingtemperature for the hot rolled sheet, etc., are observed.

The amount of C for the present invention may be that contained in theordinary low carbon cold rolled steel sheet. If it is desired to obtainlower carbon content for pursuing more excellent press formability, orlower bubbling or sagging, the molten steel may be degassed under vacuumconditionsor the material may be subjected to decarburization treatmentat the time of annealing, whereby the above object can easily beaccomplished.

1n the present invention, it is advantageous to keep the N content inthe sheet as low as possible, whereby the stabilization of the r value,the decrease of the strain aging property as well as the excellent pressformability can be obtained. As for the strain aging property, it ispossible to add such N-fixing elements as A], B, V and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENT A steel sheet obtained byconverter and subjected to the usual steps of casting, blooming and hotrolling was coiled at 700C to 730C to obtain a hot rolled coil, whichwas then allowed to stand in the air to the ordinary temperature. Thishot rolled sheet was further subjected to the cold rolling step toobtain the final thickness of 0.7mm. Thereafter the recrystalizationannealing for 700C X 1 minute and the subsequent overaging treatment for350C X 2 minutes were effected in the continuous annealing system andfurther the temper rolling was effected to the extent of 1 to 1.5%. Thetable below shows the chemical compositions of the materials (a) and (b)of the invention (analysis of the product) and the properties of thesteel sheet obtained. By way of comparison, the data of the steel platehaving a large K value are also shown in the table.

where AW atomic weight; subjecting said steel material to hot rollingfollowed by coiling the same at 600C to 800C; subjecting said coiled hotrolled strip to cold rolling to thereby obtain a cold rolled strip; andthereafter subjecting said cold rolled strip to recrystalizationannealing.

The material The material of this invention to be com- (0.7mm thick)pared (a) (b) (0.7mm thick) C 0.04 0.04 0.04 Mn 0.18 0.17 0.33Composition S 0.010 0.015 0.010 O 0.042 0.075 0.028 K 0.02 0.1 l 0.22Coiling temperature C 700 to 720 670 to 680 530 to 550 Annealing(Contin- 700CX1min. 700CX1min. 700CXlm|n. conditions uous) +350CX2min.+350CX2min. +350CX2m|n. Temper rolling(%) 1.0 to 1.5 1.0 to 1.5 1.0 to1.5 a) After temper rolling: Yield point (kglmm 19.1 19.5 22.1 Tensilestrength (kg/mm) 31.8 32.0 34.7 Elongation 44 42 r 1.60 1.5 1.12 C C V(mm) 26.02 26.25 27.21 Erichsen test (mm) 10.8 10.8 10.3 Elongation atyield point 0 0 0 b) After aging: Elongation at yield point 3.1 3.0 5.6(CX60 min.) Elongation at yield point 0 0 1.2 (room temp. X3days)Occurrence ratio of fish scaling after enameling 0% 0% 4.2% Hydrogenpass time (25C) 72 15 AW of Mn AW of Mn Remarks K= [Mn%] W X [0%] IS%]AW of Mn AWOfO where AW atomic weight; subjecting said steel material tohot rolling followed by coiling the same at 600C to 800C; subjectingsaid coiled hot rolled strip to cold rolling to thereby obtain a coldrolled strip; and thereafter subjecting said cold rolled strip torecrystalization annealing.

3. A method according to claim 2 in which said material has not morethan 0.20% by weight of manganese.

4. A method according to claim 2 in which said recrystalizationannealing is a continuous annealing with over-aging treatment.

5. A method according to claim 2 in which said recrystalizationannealing is a box annealing.

. UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO.3,912,549

DATED October 14, 1975 |NVENT0R(5) 1 Hisashi Gondo et al It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

. At Col 2, line 25, "box annealing with over-aging" should read --boxannealing or continuous annealing with overaging.

At Col. 3, line 2, "r value" should read value-.

Signed and Scaled this Thinierh Day Of January 1979 [SEAL] A nest:

DONALD W. BANNER RUTH (T. MASON Arresting Officer Commissioner ofPatents and Trademarks

-0.20 (MN%)- X(0%) X(S%) 0 AW OF 0 AW OF S
 1. A METHOD FOR THEMANUFACTURE OF A COLD-ROLLED LOW-CARBON STEEL SHEET USED FOR PORCELAINENAMEL HAVING EXCELLENT PRESS FORMABILITY AND SUITABILITY FOR ENAMELINGWHICH COMPRISES: PROVIDING A LOW CARBON STEEL MATERIAL HAVING NOT MORETHAN 0.25% BY WEIGHT OF MANGANESE, SAID STEEL MATERIAL CONTAINING MN, OAND S IN THE RELATION OF
 2. A method for the manufacture of acold-rolled low-carbon steel sheet used for porcelain enamel havingexcellent press formability and suitability for enameling whichcomprises: providing a low carbon steel material having not more than0.25% by weight of manganese; said steel material containing Mn, O, andS in the relation of
 3. A method according to claim 2 in Which saidmaterial has not more than 0.20% by weight of manganese.
 4. A methodaccording to claim 2 in which said recrystalization annealing is acontinuous annealing with over-aging treatment.
 5. A method according toclaim 2 in which said recrystalization annealing is a box annealing.